The subject matter disclosed herein relates to steam turbines and, in particular, to a diffuser with a hollow opening steam guide having a vacuum located within the hollow opening to increase pressure recovery in the flow of steam adjacent the inner steam guide surface by reducing steam flow separation adjacent the inner steam guide surface.
The low pressure section of a steam turbine typically includes several turbine blade stages and a combination exhaust hood and diffuser section, including a down flow diffuser. Functions of the exhaust hood/diffuser include the recovery of (i.e., increasing) the static pressure as the velocity of the flow of steam decreases as it enters the diffuser. Also, the diffuser acts as a turbine steam exhaust flow passage that guides the flow of steam as it exits axially from the last stage blade of the turbine and directs it radially downstream towards a condenser within the steam turbine. Similarly, the diffuser directs the flow of steam downstream into the exhaust hood. Flow diffusion commonly takes place in the initial portion of the diffuser following the last stage blade. The remainder of the diffuser functions as a collecting or guiding chamber for the steam flowing to the condenser. The diffuser steam flow channel is typically bounded by a steam flow guide and a bearing cone.
The amount of pressure recovery within a diffuser typically depends on the inlet profile of the diffuser as well as the length of the diffuser and the area ratio (i.e., the diffuser outlet-to-inlet area ratio). For a given last stage blade exit profile, there may exist an area ratio that produces the relatively greatest pressure recovery in the diffuser. However, when the area ratio is made to be greater than that which produces the relatively greatest pressure recovery, the steam flow tends to separate from the steam guide after the flow enters the diffuser. Such flow separation decreases the amount of pressure recovery in the exhaust hood/diffuser. As a result, oftentimes the area ratio is made to be less than desirable (i.e., smaller) to ensure that the flow does not separate from the steam guide and adversely affect the diffusion of the steam flow.